Conventionally, a sheet of transparent protective glass is placed over photocells, in particular photocells for use in space.
Under such circumstances, the sheet of glass is secured to the photocell by a transparent adhesive, or preferably by a thin and transparent film of polytetrafluoroethylene-fluorinatedethylenepropylene (PTFE-FEP) that is thermally fixed in the presence of a strong electric field. The sheet of glass and the fixing layer form an insulating layer that may accumulate electrostatic charge whose sudden discharge through the sheet of glass and the fixing layer, in particular when accompanied by the formation of an electric arc, runs the risk of destroying the photocell and/or of disturbing the electronic equipment of a satellite. It has thus been necessary to implement circuits for evacuating electrostatic charge from photocells designed for use in space.
In "Proceedings of European Space Power Conference" held in Madrid, Spain, 2-6 Oct. 1989. (ESA SP-294, Aug. 1989), J. W. Koch describes several types of such circuits.
Firstly, he describes a metal grid connected to ground and passing over a plurality of glass sheets for protecting photocells.
Another solution described in that article relates to connecting the outside face of the protective glass sheet to the photocell by means of a conductive transparent adhesive deposited at least over the entire outside surface of the sheet of glass and/or by a fine deposit of indium tin oxide (ITO) over the entire outside surface and sides of the photocell. Conductive deposits on the inside face of the sheet of glass can be limited to the vicinity of the connection device.
In all cases, the photocells described by J. W. Koch in ESA SP-294, August 1989, include a transparent adhesive fixing the protective glass sheet. The two examples are described in entirely separate manner and there is nothing in that article that describes or suggests the possibility of combining those two examples, and in particular of implementing a grid for evacuating electrostatic charge disposed on the outside face of the protective glass sheet and connected to circuits for collecting the photoelectric charge as produced by the photocell.
In addition, it should be observed that the photocells described in that article are designed to be carried directly on the body of a scientific satellite that is subjected to substantially constant solar illumination during a limited lifetime. Such teaching is not applicable to photocells for geostationary satellites that are periodically subjected to thermal cycles that an adhesive fixing the protective glass plate could not withstand. The photocells used for scientific satellites are extremely expensive, which makes them unsuitable for use in a telecommunications satellite.